The present invention relates to a semiconductor laser diode and a method of forming the same, and more particularly to a gallium nitride based compound semiconductor laser having a current block layer structure selectively grown for a current confinement and a method of forming the same.
Gallium nitride is larger in energy ban gap than those of indium phosphate and gallium arsenide, for which reason gallium nitride based semiconductors of In.sub.x Al.sub.y Ga.sub.1-x-y N (0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1, 0.ltoreq.x+y.ltoreq.1) may be applied to light emitting diodes such as semiconductor laser diodes for emitting a light of an wavelength in the range of green light wavelength to ultraviolet ray wavelength.
Gallium nitride based semiconductor may have either hexagonal crystal structure or cubic crystal structure. The hexagonal crystal structure is more stable in energy than the cubic crystal structure.
One of conventional gallium nitride based semiconductor laser diodes is disclosed by S. Nakamura et al. in Extended Abstracts of 1996 International Conference On Solid State Devices And Materials, Yokohama, 1996, pp. 67-69.
The conventional gallium nitride based semiconductor laser diode will be described with reference to FIG. 1. A 300 .ANG. thick undoped GaN buffer layer 102 is formed on a (11-20)-face sapphire substrate 201. A 3 .mu.m-thick n-type GaN contact layer 103 doped with Si is formed on the 300 .ANG.-thick undoped GaN buffer layer 102. A 0.1 .mu.m-thick n-type In.sub.0.05 Ga.sub.0.95 N layer 104 doped with Si is formed on the 3 .mu.m-thick n-type GaN contact layer 103. A 0.4 .mu.m-thick n-type Al.sub.0.07 Ga.sub.0.93 N cladding layer 105 doped with Si is formed on the 0.1 .mu.m-thick n-type In.sub.0.05 Ga.sub.0.95 N layer 104. A 0.1 .mu.m-thick n-type GaN optical guide layer 106 doped with Si is formed on the 0.4 .mu.m-thick n-type Al.sub.0.07 Ga.sub.0.93 N cladding layer 105. A multiple quantum well active layer 107 is formed on the 0.1 .mu.m-thick n-type GaN optical guide layer 106. The multiple quantum well active layer 107 comprises 7 periods of 25 .ANG.-thick undoped In.sub.0.2 Ga.sub.0.8 N quantum well layers and 50 .ANG.-thick undoped In.sub.0.05 Ga.sub.0.95 N barrier layers. A 200 .ANG.-thick p-type Al.sub.0.2 Ga.sub.0.8 N layer 108 doped with Mg is formed on the multiple quantum well active layer 107. A 0.1 .mu.m-thick p-type GaN optical guide layer 109 doped with Mg is formed on the 200 .ANG.-thick p-type Al.sub.0.2 Ga.sub.0.8 N layer 108. A 0.4 .mu.m-thick p-type Al.sub.0.07 Ga.sub.0.93 N cladding layer 110 doped with Mg is formed on the 0.1 .mu.m-thick p-type GaN optical guide layer 109. A 0.2 .mu.m-thick p-type GaN contact layer 111 doped with Mg is formed on the 0.4 .mu.m-thick p-type Al.sub.0.07 Ga.sub.0.93 N cladding layer 110. A p-electrode 112 is formed on the 0.2 .mu.m-thick p-type GaN contact layer 111. The p-electrode 112 comprises a nickel layer laminated on the top flat surface of the 0.2 .mu.m-thick p-type GaN contact layer 111 and a gold layer laminated on the nickel layer. An n-electrode 113 is provided on the recessed surface of the 3 .mu.m-thick n-type GaN contact layer 103. The n-electrode 113 comprises a titanium layer laminated on the 3 .mu.m-thick n-type GaN contact layer 103 and an aluminum layer laminated on the titanium layer.
All of the semiconductor layers have hexagonal crystal structure with the (0001)-face grown over the (11-20)-face sapphire substrate 201.
The above conventional gallium nitride based semiconductor laser diode has no current confinement structure, for which reason the above conventional gallium nitride based semiconductor laser diode has a relatively large threshold current.
Other conventional gallium nitride based semiconductor laser diode is disclosed by S. Nakamura et al. in Applied Physics Letters, vol. 69 (1996), p. 1477. The other conventional gallium nitride based semiconductor laser diode will be described with reference to FIG. 2. A 300 .ANG.-thick undoped GaN buffer layer 102 is formed on a (11-20)-face sapphire substrate 201. A 3 .mu.m-thick n-type GaN contact layer 103 doped with Si is formed on the 300 .ANG.-thick undoped GaN buffer layer 102. A 0.1 .mu.m-thick n-type In.sub.0.05 Ga.sub.0.95 N layer 104 doped with Si is formed on the 3 .mu.m-thick n-type GaN contact layer 103. A 0.5 .mu.m-thick n-type Al.sub.0.05 Ga.sub.0.95 N cladding layer 605 doped with Si is formed on the 0.1 .mu.m-thick n-type In.sub.0.05 Ga.sub.0.95 N layer 104. A 0.1 .mu.m-thick n-type GaN optical guide layer 106 doped with Si is formed on the 0.5 .mu.m-thick n-type Al.sub.0.05 Ga.sub.0.95 N cladding layer 605. A multiple quantum well active layer 707 is formed on the 0.1 .mu.m-thick n-type GaN optical guide layer 106. The multiple quantum well active layer 707 comprises 7 periods of 30 .ANG.-thick undoped In.sub.0.2 Ga.sub.0.8 N quantum well layers and 60 .ANG.-thick undoped In.sub.0.05 Ga.sub.0.95 N barrier layers. A 200 .ANG.-thick p-type Al.sub.0.2 Ga.sub.0.8 N layer 108 doped with Mg is formed on the multiple quantum well active layer 707. A 0.1 .mu.m-thick p-type GaN optical guide layer 109 doped with Mg is formed on the 200 .ANG.-thick p-type Al.sub.0.2 Ga.sub.0.8 N layer 108. A 0.5 .mu.m-thick p-type Al.sub.0.05 Ga.sub.0.95 N cladding layer 710 doped with Mg is formed on the 0.1 .mu.m-thick p-type GaN optical guide layer 109. A 0.2 .mu.m-thick p-type GaN contact layer 111 doped with Mg is formed on the 0.4 .mu.m-thick p-type Al.sub.0.05 Ga.sub.0.95 N cladding layer 710. The 0.2 .mu.m-thick p-type GaN contact layer 111 has a ridge-shape. A p-electrode 112 is formed on the top portion of the 0.2 .mu.m-thick p-type GaN contact layer 111. The p-electrode 112 comprises a nickel layer laminated on the top flat surface of the 0.2 .mu.m-thick p-type GaN contact layer 111 and a gold layer laminated on the nickel layer. A silicon oxide film is formed which extends on the sloped side walls of the ridge portion of the 0.2 .mu.m-thick p-type GaN contact layer 111 and also on the flat base portions of the 0.2 .mu.m-thick p-type GaN contact layer 111 as well as on side walls of the above laminations of the 3 .mu.m-thick n-type GaN contact layer 103, the 0.1 .mu.m-thick n-type In.sub.0.05 Ga.sub.0.95 N layer 104, the 0.5 .mu.m-thick n-type Al.sub.0.05 Ga.sub.0.95 N cladding layer 605, the 0.1 .mu.m-thick n-type GaN optical guide layer 106, the multiple quantum well active layer 707, the 200 .ANG.-thick p-type Al.sub.0.2 Ga.sub.0.8 N layer 108, the 0.1 .mu.m-thick n-type GaN optical guide layer 109, the 0.4 .mu.m-thick p-type Al.sub.0.05 Ga.sub.0.95 N cladding layer 710 and the 0.2 .mu.m-thick p-type GaN contact layer 111. An n-electrode 113 is provided on the recessed surface of the 3 .mu.m-thick n-type GaN contact layer 103. The n-electrode 113 comprises a titanium layer laminated on the 3 .mu.m-thick n-type GaN contact layer 103 and an aluminum layer laminated on the titanium layer.
All of the semiconductor layers have hexagonal crystal structure with the (0001)-face grown over the (11-20)-face sapphire substrate 201.
The above ridge structure of the 0.2 .mu.m-thick p-type GaN contact layer 111 might contribute any current confinement for reduction in threshold current. Since, however, a contact area between the p-electrode and the 0.2 .mu.m-thick p-type GaN contact layer 111 is small, a contact resistance of the p-electrode to the 0.2 .mu.m-thick p-type GaN contact layer 111 is relatively large.
Whereas the above ridge structure of the 0.2 .mu.m-thick p-type GaN contact layer 111 is defined by a dry etching process, this dry etching process may provide a damage to the semiconductor layers.
The use of this dry etching process results in complicated fabrication processes for the laser diode.
In the above circumstances, it had been required to develop a novel gallium nitride based compound semiconductor laser and a method of forming the same.